1. Field of the Invention
The present invention relates to a combined cycle power generation plant capable of setting a steam generated from an exhaust gas heat recovery boiler to a proper temperature and supplying the steam to a steam turbine plant while supplying the steam generated from the exhaust gas heat recovery boiler to a gas turbine plant as a cooling steam, and also relates to a cooling steam supply method for the combined cycle power generation plant.
2. Description of the Related Art
In recent years, a study and development for obtaining high power and achieving high heat efficiency has been made in a combined cycle power generation plant. With the study and development, there has been made a plan to raise a combustion gas temperature of at least a portion of a gas turbine inlet from a temperature of 1300.degree. C., obtained in the prior art, to a temperature of 1500.degree. C. or more.
In the case of creating a high temperature of the combustion gas of the gas turbine inlet, for example, a high chromium steel has been conventionally used as a component of a gas turbine plant, and part of the compressed air from an air compressor has been supplied to the component of the gas turbine plant as a cooling medium. However, in the prior art as described above, the strength of the component has been close to its limit. For this reason, in order to discover a cooling medium substituting for the compressed air used in the prior art, it has been attempted to study and develop a new cooling medium to be supplied to the components of the gas turbine plant, and steam has been selected as one of the cooling medium. A combined cycle power generation plant which takes advantage of steam cooling has been already disclosed in, for example, Japanese Laid-Open Patent Publication Nos. 5-163960 and 6-93879.
Steam has a higher specific heat as compared with compressed air and is adapted to an absorption of heat generated in components, for example, in a gas turbine stationary blade and a movable blade, accompanying with high temperature of the gas turbine plant. However, each of the gas turbine stationary blade and the movable blade has a structure in which a complicatedly meandering narrow passage is defined in the interior of these blades. For this reason, if impurities such as silica or the like are contained in a steam passing through the above passage, unbalanced cooling occurs because of the possibility of clogging the passage with silica or the like. As a result, these blades are broken down due to thermal strain accompanying the unbalanced cooling. Therefore, cooling steam is required having a high cleanliness factor.
Further, in the case where a cooling steam is supplied to components of the gas turbine plant, it is necessary to provide a steam supply source which can supply a steam of proper temperature. If not so, the component of the gas turbine plant generates an excessive thermal stress resulting from the difference in temperature between a combustion gas as a driving fluid and these components, which difference may result in a possibility that these components are broken down. For this reason, in the components of the gas turbine plant, a steam supply source, which can supply a steam of proper temperature, is securely required.
On the other hand, with a temperature of the gas turbine plant being high, a steam supplied from the exhaust gas heat recovery boiler to a steam turbine plant also has a high temperature. In this case, if the steam temperature is too high, an excessive thermal stress is generated in the steam turbine plant, and as a result, it becomes difficult to maintain a material strength of the components of the steam turbine plant. For this reason, in the steam turbine plant, it is necessary to provide a steam supply source which can supply a steam of a proper temperature.
As described above, in the combined cycle power generation plant, a first high pressure superheater of the exhaust gas heat recovery boiler is selected and set as a steam supply source, taking into consideration the cleanliness of cooling steam, supply of proper temperature steam, and technical matters indispensable to the gas turbine and steam turbine plant. As one example, a combined cycle power generation plan as shown in FIG. 6 has been already proposed.
The combined cycle power generation plant shown in FIG. 6 has an arrangement in which a gas turbine plant 1 and a steam turbine plant 2 are combined by a common rotary shaft 3 and an exhaust gas heat recovery boiler 4 is located independently from these plants.
The gas turbine plant 1 includes a generator 5, an air compressor 6, a combustor 7 and a gas turbine 8. Air AR sucked by the air compressor 6 is made into a high pressure compressed air, and is guided to the combustor 7. In the combustor 7, a fuel is added to the compressed air so that a combustion gas is generated, and then, the combustion gas is expanded by the gas turbine 8, thus the generator 5 is driven by the power generated in the above manner.
The steam turbine plant 2 includes a high pressure turbine 9, an intermediate pressure turbine 10, a low pressure turbine 11 and a condenser 12. An exhaust steam, after being expanded by the high pressure turbine 9, is led to a reheater 13 of the exhaust gas heat recovery boiler 4 and is superheated therein. Then, the exhaust steam is led to the intermediate pressure turbine 10 and is expanded as a reheat steam. Further, the exhaust steam is again expanded by the low pressure turbine 11, and thereafter, is condensed into a condensate by the condenser 12. The condensate is supplied as a feed water to the exhaust gas heat recovery boiler 4 via a pump 100.
Meanwhile, the exhaust gas heat recovery boiler 4 is provided with a third high pressure superheater 14, the reheater 13, a second high pressure superheater 15, a first high pressure superheater 16, a high pressure evaporator 18 including a high pressure drum 17, an intermediate pressure superheater 19, a high pressure economizer 20, a low pressure superheater 21, an intermediate pressure evaporator 23 including an intermediate pressure drum 22, an intermediate pressure economizer 24, a low pressure evaporator 26 including a low pressure drum 25, and a low pressure economizer 27. These components or elements are arranged in order from an upstream side toward a downstream side along a flow of an exhaust gas G of the gas turbine plant 1, and steam is generated through the heat exchanging operation between each heat exchanger and the exhaust gas G.
Specifically, in the exhaust gas heat recovery boiler 4, a feed water supplied from the condenser 12 of the steam turbine plant 2 via the pump 100 is preheated by the low pressure economizer 27 and is led to the low pressure drum 25. Then, by taking advantage of a difference in density of drum water, the feed water is circulated through the low pressure evaporator 26 to generate steam, and the generated steam is supplied to the low pressure turbine 11 via the low pressure superheater 21.
The low pressure economizer 27 leads part of the feed water, which is diverted (divided) on an outlet side of the economizer 27, to the low pressure drum 22 by a low pressure pump 28 and the intermediate pressure economizer 24. Due to a difference in density of drum water, a part of the saturated water is circulated through the low pressure evaporator 23 to generate steam, and then, the generated steam is supplied to the gas turbine plant 1 via the intermediate pressure superheater 19 so as to cool the components of the gas turbine 8.
Further, the low pressure economizer 27 leads the remaining feed water to the high pressure drum 17 by a high pressure pump 29 and the high pressure economizer 20. Then, the remaining saturated water is circulated through the high pressure evaporator 18 to generate steam, and the generated steam is led to the first high pressure superheater 16.
This first high pressure superheater 16 includes a steam pipe 30 for leading steam to the second high pressure superheater 15, and a bypass pipe 32 between which a bypass valve 31 is interposed. Steam passed through the bypass pipe 32 is joined together with a superheated steam generated by the second high pressure superheater 15, and after the temperature of the steam has been decreased to a proper temperature, the steam is supplied to the high pressure turbine 9 of the steam turbine plant 2 via the third high pressure superheater 14.
As described above, in the known combined cycle power generation plant, in the case where steam is supplied from the exhaust gas heat recovery boiler 4 to the high pressure turbine 9, the first high pressure superheater 16 is set as the steam supply source. When the steam generated from the first high pressure superheater 16 is made into a superheated steam by the second high pressure superheater 15, the steam temperature is decreased by the bypass pipe 32, and then, the superheated steam having a proper temperature is supplied from the third high pressure superheater 14 to the high pressure turbine 9.
Moreover, when supplying a cooling steam to the components of the gas turbine 8, in the exhaust gas heat recovery boiler 4, a superheated steam generated by the intermediate pressure superheater 19 and an exhaust steam of the high pressure turbine 9 are joined together, and then, the joined steam is supplied to the gas turbine 8 so that the strength of the gas turbine members can be maintained so as to adapt to high temperature of a combustion gas on an inlet of the gas turbine 8. Further, a steam, which cooled the components of the gas turbine 8, is then led to the intermediate pressure turbine 10 together with a reheated steam of the reheater 13.
Meanwhile, in the combined cycle power generation plant shown in FIG. 6, during the start-up operation, the steam is still not generated from the exhaust gas heat recovery boiler, and for this reason, a cooling steam cannot be supplied to the gas turbine 8 from the intermediate pressure superheater 19 and the high pressure turbine 9. Thus, in order to cool the components of the gas turbine 8, there is the following plan for making use of the steam remaining in the high pressure drum 17 of the exhaust gas heat recovery boiler 4. Specifically, in this case, the exhaust gas heat recovery boiler 4 can make use of a residual heat of the first high pressure superheater 16, the second high pressure superheater 15 and the third high pressure superheater 14. Therefore, as shown in FIG. 7, an outlet side of the first high pressure superheater 16 is provided with a cooling steam pipe 34 which is arranged parallel to the bypass pipe 32 and includes a control valve 33.
The residual steam of the high pressure drum 17 is led to the first high pressure superheater 16 so as to be superheated, and then, part of the residual steam is guided to the second high pressure superheater 15 and the first high pressure superheater 14 while the remaining steam thereof is led to the cooling steam pipe 34. Subsequently, the two flows of steams are joined together on the outlet side of the third high pressure superheater 14, and a high-temperature portion of the gas turbine 8 is temporarily cooled by the joined steam. When the gas turbine plant 1 is in a high-load state, the components of the gas turbine 8 are cooled by the joined steam of the intermediate pressure superheater 19 and the high pressure turbine 9.
As described above, in the combined cycle power generation plant shown in FIG. 6, the known plan mentioned above has been performed such that a steam of a proper temperature is supplied from the exhaust gas heat recovery boiler 4 to the high pressure turbine 9 during the rated operation. However, when the gas turbine plant 1 is in a state of a partial load operation, the exhaust gas G supplied from the gas turbine 8 to the exhaust gas heat recovery boiler 4 is further increased in its temperature.
In general, in the case where the partial load operation of the gas turbine plant 1 is carried out, as shown by a broken line in FIG. 8, a temperature of the exhaust gas G rises. In contrast to the rising of the temperature of the exhaust gas G, the steam temperature of the first high pressure superheater 16 is substantially constant as shown by a dotted chain line in FIG. 8. On the other hand, the steam temperature of the second high pressure superheater 15 becomes high as shown by a solid line in FIG. 8. The steam temperature of the third high pressure superheater 14 rises, not shown, like the second high pressure superheater 15. In this case, the exhaust gas heat recovery boiler 4 sets the superheated steam of the third high pressure super heater 14 at a proper temperature and supplies it to the high pressure turbine 9. Thus, when supplying the superheated steam of the first high pressure superheater 16 to the third high pressure superheater 14 via the bypass pipe 32, a bypass steam flow rate is increased as shown by a solid line in FIG. 8. For this reason, a heat exchange quantity of the second high pressure superheater 15 is increased as the exhaust gas reaches high temperature. However, a steam quantity of any heated object remarkably decreases, and during the heat exchange, an excessive thermal stress is generated due to a biased temperature distribution. As a result, a problem is caused such that a heat transfer pipe is burned or broken down.
On the other hand, the combined cycle power generation plant shown in FIG. 7 is constructed as follows. Specifically, during a start-up operation, the outlet side of the first high pressure superheater 16 is provided with the cooling steam pipe 34 which is arranged parallel to the bypass pipe 32. Due to the steam remaining in the high pressure drum 17, the steam is led to the first high pressure superheater 15, which is used as a cooling steam supply source. Further, part of the steam is supplied to the cooling steam pipe 34 while the remainder thereof is supplied to the third high pressure superheater 14 via the second high pressure superheater 15. Subsequently, both the steam flows are joined together on the outlet side of the third high pressure superheater 14, and the joined steam is supplied to the gas turbine 8 so as to cool the components of the gas turbine 8.
However, even during the start-up operation, for example, when the gas turbine plant 1 is in a hot start or a very hot start state, a residual heat of each heat exchanger is still at a high temperature, and for this reason, there sometimes arises a case where the temperature of the cooling steam exceeds a proper cooling steam temperature of the gas turbine 8. In order to realize a proper temperature of the cooling steam, as shown in FIG. 7, it is necessary to locate a steam generating apparatus 35, which generates a steam having a relatively low temperature, on an inlet side of the gas turbine 8. However, this arrangement is not advantageous when the cost of facilities is considered.
In FIG. 7, the outlet side of the gas turbine 8 is provided with a first bypass pipe 12a connected to the condenser 12. Further, the inlet side of the high pressure turbine 9 is provided with a second bypass pipe 12b connected to the condenser 12.
As described above, in the known combined cycle power generation plants shown in FIGS. 6 and 7, the following plan has been made. Specifically, the first high pressure superheater 16, which generates steam having a stable temperature with respect to a load variation as shown in FIG. 8, is set as a steam supply source, and steam having a proper temperature is supplied to the high pressure turbine 9 therefrom while a cooling steam having a proper temperature is supplied to the gas turbine 8. Considering the details, however, the conventional combined cycle power generation plants have various problems as described above, and it is required to achieve improvements for sufficiently coping with the high temperature of the gas turbine plant.